Heat-treating high-speed tool steel



Aug. 21, 1945. FQ A. ENDRESS 2,383,299 HEAT TREATING HIGH-SPEED TOOLSTEEL Filed sept. e, 1943 Patented Aug. 21, 1945 HEAT-TEEATIN GHIGH-SPEED TOOL STEEL Frederick A. Endress, Detroit, Mich., assignor toTuff-Hard Corporation, Detroit, Mich., a corporation of` MichiganApplication September 6, 1943, Serial No. 501,394

2 Claims. (Cl. 148-421.55)

This invention relates to the heat treatment of high-speed tool steelsand has for its principal object the provision of certain step or stepsof operation in the heat treatment of high-speed tool steels that willresult in a cutting tool of superior characteristics.

Objects of the invention include the provision of a method ofheat-treating high-speed tool steel that will result in a tool ofsuperior hardness and toughness; the provision of a method ofheat-treating high-speed tool steel by the practice of which a cuttingtool having a superior cutting edge will be provided; the provision of amethod of heat-treating high-speed tool steel by the practice of which acutting tool may be provided in most cases capable of producing a muchsmoother surface on a part machined thereby than has heretofore beenpossible with highspeed tool steels treated in a conventional manner,and at the same time the tool will embody a superior hardness andtoughness as compared to high-speed tools heat-treated in a conventionalmanner; the provision of a method of heattreating high-speed tool steelin which cutting tools formed therefrom will Wear longer than such toolsformed from high-speed tool steel heat-treated in the usual manner; theprovision of a method of heat-treating high-speed tool steel by means ofwhich a more uniform disper- -sion of the undissolved carbides can oftenbe obtained; and the method of heat-treating highspeed tool steel by thepractices of which the steel may be brought to a desired quenchingtemperature in a shorter period of time than is possible. byconventional methods, whereby to minimize damage to the steel because ofthe high temperature and enabling the beneficial results above.

enumerated ,to be obtained.

The above being among the objects of the present invention the sameconsist in certain novel steps and combinations of steps of operation toillustrating the time and temperatures preferably employed in thetreatment of high-speed tool"V steeland including the steps of thepresent invention.'

A The present invention is particularly directed toso-calledhigh-speedftool steel and particularly tothpse'having tungsten;,m0,1ybdenun'i,` or some u ivjaflentelement alloyed therewith,,v'hich`t9' or heat treatments in the neighborhood of from 900 F. to 1125 F.after hardening.

It is well understood by those skilled in the art that such high-speedtool steel should be hard in order to provide the proper character ofcutting edge and be capable of maintaining such u cutting edge inservice, and yet it should have the characteristic of being tough sothat the cutting edge will not break down under impact or the toolitselfbreak under the load of the cutting operation. In my copendingapplication for Letters Patent of the United States for improvements inprocess for heat-treating steel, Serial No. 370,577,l led December 18,1940, which has matured into Patent Number 2,364,893, I have disclosed amethod of heat-treating high-speed tool steel by the practice of whichtool steels having a uniform hardness of better than sixtyfive RockwellC has been obtained with a uniform toughness or tensile strength of morethan 500,000 pounds per square inch. Except for the specific steps ofoperation disclosed herein for bringing the high-speed tool steel fromthe preheat temperature to the quenching temperature and whichconstitutes the gist 'of the present invention, the remaining steps ofoperation disclosed in my prior application above identified may be, butare not necessarily, followed in carrying out the complete sequency ofheat-treating operations required to bring a high-speed tool steel partto its finally hardened condition in accordance with the presentinvention. In other words, it will be understood, for the purpose ofexplanation only, that the steps of operation, outside of thoseconstituting the present invention, required to complete the fullheat-treatment of a piece of high-speed tool steel, may be those stepsdisclosed in my prior application above identified or may be equivalentconventional steps such as are employed in the industry.

It has been generally understood in the art that in bringing high-speedtool steel to a quenching temperature, the steel must be brought to atemperature from about 2150 F. 'to 2450 F., depending uponthecomposition of the material from which the particular tool is made,from the preheat temperature of around 1500 F. to 1650 F., as quickly aspossible and held at this high temperature] for only substantially asufcient length of time to insure the tool being substantially uniformlyheated throughout, then immediately quenched, in order to obtain thede.- sired hardness'without burning thevtool. This heati'x'ig` of thetool steel'parts from the preheat temperature to the quenchingtemperature is contool at the preheat temperature is placed in a furnacemaintained at a temperature sufficiently in excess of the desiredquenching temperature of the tool to provide a head of heat capable ofraising the tool quickly to approximately the quenching temperature,andA when the -part -has remained in the high-heat furnace for asufficient length of time to raise-itv approximately to the quenchingtemperature, Vit is immediately withdrawn and placed in a second furnacemaintained substantially -at and 'preferablywith a small head of heatbeyond the quenching-temperature desired for the tool being treated. Itis allowed to `remain in the secondfurnace onlyfor a sufiicient lengthof time to insure the part being substantially uniformly heatedthroughout and the desired carbide transformations having taken place,upon which it is immediately withdrawn andA quenched., I have found thatby following out this procedure high-speed tool steel parts may beraised to a quenching temperature which is substantially uniformthroughout the part in a much shorter time than in accordance withconventional 'practices and eliminates to a greateX- tent the danger ofburning the corners or edges of the parts thus being heated.

By the practice of the steps of the present invention, not only do theheat-treated highspeed steel parts have a hardnes and toughness equal'to that obtained by following out the method of my copendingapplication above identifled, but in fact produce high-speed steel partshaving even a superior quality of toughness and hardness. Additionally,I have found that highspeed tool speed cutting tools produced inaccordance with the present invention are usually capaods or by themethod disclosed in my copending application above identified. While theexact reason for this superiority may not be positively known, itappears from microphotographs 0f sections of high-speed tool steelheat-treated in accordance with the present invention that thesuperiority may result to a great measure from a breakingdown of theundissolved carbldes into smaller particles which are more closelyspaced 4and more uniformly vdispersed throughout the mass of the toolthan occurs in such high-speed tool steel when heat-treated inaccordance with conventional practice or by the practice disclosed in mycopending application above referred to.

By following out the practices of the present invention, however, it isfound that the particles of undissolved carbide are usually smaller thanprovided in the conventional or-older practice and that these carbidesare usually more uniformly dispersed through the material of the tool.

forming a matrix therefor and are more closely arranged with respect toeach other` than 'the larger particles resulting in the older practice.

When this occurs a finer cutting edge is obtained,I

resulting in many cases in smoother work cut by the tool.

I'n the accompanyingdrawingl have graphically illustrated the preheat,the high-temperathe preheat.

ture heat, and the quench embodied in a preferred series of steps inheat-treating high-speed tool steel in accordance with the presentinvention, it being understood that the gistv of the present inventionis the high-temperature heat or those `steps of operations employed inbringing a high-speed tool steel part from the preheatingl temperatureto the quenching temperature. The

preheating and theV quenching steps, while pref.

erably -substantially the same as tha'tdisclosed in the graph, are notnecessarily restricted thereto, as any satisfactory preheating step,quenching step, ,drawingand other steps may be employed, and theresulting tool will be found to be beneted by the practices of thepresent invention.

Therst step shown in the graph, in carrying out'the heat treatment ofhigh-speed tool steel, including the steps of the present invention isWhile, as above described, the preheat may be carried out in anysuitable or conventional manner, I find it preferable to preheat thesteel from vroom temperature by ,applying radiant heat thereto. This ispreferably carried out by placing the steel piece being processed in aceramic retort which is preferably externally heated and constructed touniformly conduct the Y heat to the steel by radiation. The steel isthus initially vheated to a temperature of approximately at least 1000F. but lessv than the iron carbon critical AC3 range, -as indicated inthe graph by that portion of the curve designated by the numeral I2.sirable to heat the steel in this first preheat to a temperature of from1100" to 1250 F., and preferably to about 1200 F. and this heat ispreferably within a minimum time of about thirty minutes for allstandard sections up to 3A square. This minimum time is preferablyincreased approximately about ten minutes for each 1/8` square increasein dimension. Preferably, the time control during this heating operationis had by shielding the steel being heat-treated in any suitablemannerrso as to permit the absorption Y of heat by the steel at acertain predetermined rate, whereby to uniformly increase thetemperature of the steel material upto at least 1000 F.

and to the preferred temperature as mentioned above within thepredetermined time.

Next the high speed tool steel is allowed to soakl at this temperaturefor a period of time varying from about thirty minutes to four hours orthereabouts, indicated in the graph by that portion of the curvedesignated by the numeral I4, thereby permitting an isothermal change totake place in.

the iron structuravthis change resembling in some respects the changewhich takes place in the standard spheroidizing process and it alsoallows iron-carbon critical (AC3) range, and preferably to about 1500 F.and preferably not exceeding a maximum of 1650 F., but preferably withina minimum of about fifteen minutes for-sections up to three-fourths (3A)inch square, which time is increased approximately ve minutes for eachone-eighth (1/8) inch square increase in dimension. This step, which isillustrated in the graph by that portion of the curve designated by thenumeral l5, may be carried out in any suitable or lconventional manner'vbut I prefer to uniformly heat the steel by placingthe same in a heatedceramic retort or other suitable retort providing fora'A radiation ofheat from the retort to the In most instances, I nd it del range, but inthis step, the steel is never allowed to soak at this temperature ofabout 1550? F. asl

it is immediately raised to the quenching temperature` as quickly aspossible, preferably by means of radiant heat or by induction heatingbut broadly by any suitable or conventional means which will uniformlyheat the steel to the required temperature Within the time prescribed.Preferably the steel being treated is raised to a quenching temperature.in a carbonaceous gas, in a vacuum, or in any atmosphere which will notsupport combustion, but this is not essential.

The next Vtwo steps constitute the gist of the present invention andconsists, first inplacing the steel in a furnace or the like having amaterial head of heat above the quenching temperature, and second, thentransferring it to a furnace or the like substantially at the quenchingtemperature, the two steps being combined to quickly vraise i the steelto the quenching temperature.

The ideal time for heat treating the steel from the preheat temperatureto the quenching operation is preferably .from less than aminute to afew minutes, depending upon thesize and shape of the piece of work beingtreated. As an example, in a tool bit of square it would require aminimum of about up to two minutes, while a square bit may require abouttwo and onehalf minutes, and not more than approximately three minutes.

Quenching temperatures vary for different typesof high-speed tool steelmaterial from 2150 F. to 2450* F., and in order to raise the steel piecebeing treated to the desired quenching temperature it is preferable toheat thel first furnace, ceramic 'retort or other container used inthese two steps to a temperature which will provide a head of heat atleast 200 F., and preferably 300 F., or more above the desired quenchingtemperature, and preferably at 2650 F. to 2950 F., probably averagingfor most sizes and material, about 2800 F. Higher temperatures may beadvantageously Aemployed but under present designs in the furnace artthese are about the highest temperatures that can be consistentlymaintained without undue destruction` of the furnaces. l

In all cases, independent of the size and/or analysis of the steel'piece being treated, it is preferable to heat--the steel material in asshort a a temperature of from` temperature, and subsequently heldat thisprescribed ideal high temperature for a period of time suilicient toallow, the desired degree of carbide transformation to take place.

For example, in heat treating a one-half (1/2) inch square sectionaccording to my process, the ideal temperature whenthe carbide formingelements go into solution to obtain maximum working characteristicscombined with strength and hardness, for a steel such as 18-4-1 Vhighspeed tool steel, is approximately 2400 F. More particularly this highspeed tool steel is thus first heated to a degree as near to 2400 F. asis practical in about forty (40) seconds with an initial head of heat ofpreferably from F. above -the said .2400 F. For example, this furnacemay be heated to from about 2700 F. to about 2950 F.,'as indicated bythe dotted extensionof that portion' of the curve designated by thenumeral I6 in the graph which portion of the curve designated by thenumeral y I8. After the piece has attained this temperaturesubstantially uniformly throughout it is held in this last furnace fromzero seconds to about one and one-half minutes, depending upon theresults desired and the size of piece. For the ortime as is possibleto'or/as near as possible to the ideal high temperaturef at which thecarbides tend to go into solution. 'In most high speed tool steels, Ihave found that this ideal point of transformation 'is near the highesttemperature to which the steel can be heated without surfaceor interiordisintegration, and in all materials can be found by experiment at ornear the-ideal for each analysis.

High speed tool steels will all harden to a degree at a pointconsiderably below this ideal high temperature, but if the timeforheating the steel increases substantially beyond the limits prescribed,there is a noticeable structural characteristic that is inferiorTherefore, to give the treated `material the highest working eiciencycombined with strength and hardness, itis necessary to` eliminate so faras is practical, any partial dinary run of work the material willusually be held in the holding furnace for from thirty (30) seconds toone minute after attaining a, uniform temperature throughout. In thisconnection it,

will be appreciated thatthe longer the piece is` held in this lastfurnace the harder will bethe resulting tool, and the shorter the timethe tougher the tool. It will be noted that under proper operatingconditions, the second furnace will raise the temperature of the steelmore slowly throughout its entire mass to or near the desired hightemperature within the above prescribed time interval and will cause aproper vtransformation of the carbides to take place.

For tool steels of a different analysis, the above desired hightemperature will be varied within the limits described above, and forsteel pieces of different sizes and shapes, the time element willbevaried accordingly but substantially within the limits prescribed. Forsuch steel pieces, which are smaller in cross-section than the specieexample described above, the time will be lesser, and for larger steelpieces, the time will be greater, as will be'appreciated.

The above described high heat is: preferably accurately controlled byintroducing the material being heat treated which has been preheated toa'temperature of about 1550? F. in a high temperature furnace whoseinitial temperature ranges from approximately 2650 F. to 2950 F. Thematerial is left in this first furnace for periods of time varying fromapproximately twenty in a furnace seconds to one minute and possibly notexceeding a total time of two minutes. Due to the excessive heat in thisfurnace the steel material is very rapidly heated to a degree very closeto the predetermined ideal high temperature. In order to more accuratelycontrol the temperature of the work when same is to be quenched, I findit desirable to remove the material from this first high temperaturefurnace and immediately introduce same into asecond furnace identifiedbroadly as a holding furnace. This second furnace is preferably heatedto an initial temperature of a maximum of approximately 50 F. above thedesired quenching temperature and thus there is only a slight head ofheat in this second furnace, thus permitting the temperature of thoseportions of the steel not already at. the desired temperature to be moreslowly raised to the ultimatequenching temperature and be uniformlyheated throughout its mass. I find it desirable to leave the steelmaterial being heat treated in this second furnace for predeterminedtimed periods varying in accordance with their size and shape,

as previously mentioned.

The quenching temperatures of this class of steel 'materials varysomewhat, depending upon the analysis of the steel alloy being processedand also depending upon the application of the cutting tool. I find thatusually the desirable quenching temperatures vary from approximately2l50 F. to 2450 F and in all cases the rst high temperature furnace isheated to a degree so that it will give up heat to the material to raisethe temperature of the material to approximately the desired quenchingtemperature, preferably to a degree only or 50 F. less than the ultimatetemperature in the prescribed time, and the secondrfurnace gives up heatto the steel at a slower rate, so that a small variation in the timingduring the heating of the steel in the second furnace will notsubstantially vary the ultimate tempera'- ture of the work. This way,the work, or thevarious types of tool steel pieces being'processed, iswithdrawn from the second furnace and immediately quenched. Thisoperation results in a more uniform product 'and eliminates, to a largeextent, non-uniformity in results due to production variations intiming, caused by the human element involved in carrying out my process.

When'the steel material being treated has reached the predeterminedquenching temperature and has been. held at that'te'mperature for thedesired length of time, the same is then quenched. Whilethe quenchingmay be done in any suitable manner in the broader aspects of theinvention, preferably it is introduced into a quenching mediumsubstantially in a manner as described in the aforesaid copendingapplication. The material is thus preferably quenched from the quenchingtemperature to a temperature of approximately 900 F., as shown bythatportion of the curve in the graph indicated at 20', within a timeinterval varying from five (5) to fifteen (15) seconds. .Next the steelis preferably somewhat more slowly quenched to approximately 400 F., asshown by that portion of the curve in the graph indicatedat 22, at whichpoint same may be soaked at this temperature for variable time intervalsand then quenched to room 'temperature, orbrought directly to roomtemperajecting the steel ture particularly with the less densematerials, as optionally indicated in the graph. Subsequent tothisquenching operation the steel is then subjected to suitable orconventional drawing heats commencing at either 400 F. or roomtemperature, or in substantial accordance with the drawing operations asdescribed in my aforesaid copending application, to bring the steel to'its finally heat treated condition.

From the above it will be appreciated, as previously described, that ahigh-speed tool steel part which has been heat-treated according to thesteps'of the present invention and which has been subsequently properlydrawn not only results in a part of maximum hardness and toughness, butone in which in most cases the cutting edge thereof will include smallerand more evenly able to produce smoother nishes on metal parts finishedthereby than and heat-treated in accordance with the conventionalpractice and, at the same time. the cutting edge will maintain itssharpness and smoothness for a greater length `of time in service.

What is claimed is:

1. A process of heat treating a preheated highspeed tool steel having asecondary hardness range upon drawing, comprising the steps of heatingof said steel from a starting temperature of about 1500 F, and not inexcess of 1650? F. ap-

oximately to a quenching temperature of 2150" to 2450 F. in a furnacehaving a sufiicient head of heat to raise the temperature of an outerlayer of the steel to said temperature within a time period of fromabout twenty seconds to about one and one-half minutes, immediatelytransferring said steel into a second furnace of a temperatureapproximately the same ias the quenching temperature and holdingsaidsteel in said furnace for a period of time of from about twentyseconds to two minutes, and then quenching the steel.

2. In a process for heat treating high-speed tool steel having asecondary hardness range upon and which has been preheatedto atemdrawing, perature of about 1500" F. and not exceeding about 1650 F.,a method of controlling the heating of the steel to a quenchingtemperature of 2150 to 2450" F. consisting in the steps of subto afurnace temperature of several hundred degrees F. in excess of thedesired quenching temperature for a period of. time of from about twentyseconds to about one and one-half minutes in order that the carbidetransformations will take place as near to the desired quenchingtemperature as ispo'ssible, removing said steel from said furnace withinsaid time limit and as soon as the temperature of an outer layer thereofhas been raised to approximately the desired quenching temperature andimmediately Itransferring said steel at the last-mentioned temperaturethereof to a second furnace whose temperature is substantially the sameas said quenching temperature, and holding the steel in said secondfurnace for such length of time as to cause the desired degree ofcarbide transformation to take place in said steel, and then quenchingsaid steel.

FREDERICK l A. ENDRESS.

is possible with a high-4 speed tool steel made from the same materialA

